The patent No. FR-A-No. 2,559,694 described a heating appliance of the type defined above, more specifically a portable soldering iron, incorporating a rechargeable reservoir for the fuel gas, for example butane. Such an appliance comprises, in the direction of circulation of the fuel gas:
a double member for controlling the gas stream coming from the rechargeable reservoir, composed, on the one hand, of a valve controlled in terms of opening or closing by a suitable mechanism and, on the other hand, of a member for adjusting the gas flow leaving the valve when the latter is open;
an injector ejecting a gas jet from the gas stream coming from the double control member;
a means for driving primary air by means of the gas jet, to form a combustible mixture circulating in a pipe;
a naked-flame burner located at the end of the preceding pipe and having a member for the retention of the said flame;
a junction tube connecting the outlet of the naked-flame burner to the inlet of the catalytic burner, having orifices both for the passage of the secondary air necessary for the naked flame and for igniting the flame burner; these orifices are associated with a shutter flap movable in translational motion and controlled by the user;
an actual catalytic burner comprising a refractory, that is to say heat-resistant, core through which extends in the direction of the combustible mixture, from its inlet face to its outlet face, a plurality of conduits, the inner surface of which is covered with a suitable combustion catalyst; and
a cap equipped with burnt-gas discharge ports and closing the outlet of the catalytic burner; this metal cap is in a heat-exchange relationship with the burner and, because of a suitable shape, serves as a soldering bit.
Such an appliance is ignited in the following way.
By actuating the gas-flow adjustment member and, separately, the control of the valve, the naked-flame burner is supplied with a mixture rich in fuel gas.
By opening the flap shutting off the orifices in the junction tube, on the one hand the ignition of the naked-flame burner is assisted by the intake of secondary air, and on the other hand, the same burner can be ignited, for example, by the flame of a lighter introduced into one of these orifices.
The inlet face of the refractory core of the catalytic burner is heated by the end of the naked flame.
When the refractory core is at the desired temperature, the user commands the closing of the orifices in the junction tube by means of the shutter flap. The naked-flame burner is thus extinguished, and the mixture feeding the latter circulates from the outlet of the naked-flame burner to the inlet of the catalytic burner via the junction tube.
Because the refractory core is at the working temperature of the catalytic material, the catalytic burner comes into operation. The combustion smoke and the thermal contact of the metal cap with the catalytic burner heat the bit, which is then ready for use.
Such a method of ignition has various serious disadvantages.
First of all, the presence of a naked-flame burner in the path of the fuel gas from the injector towards the catalytic burner makes the construction of the appliance especially complex.
Subsequently, the system with a shutter flap for the junction tube can lead to various incorrect operations.
The user may not be able to open this flap and may attempt ignition at the ports for discharging the burnt gases in the cap. He will not succeed, but the combustible mixture will nevertheless continue to escape from the appliance.
The user may close this shutter again too soon, at a moment when the catalytic burner is not sufficiently hot to ignite; or he will have to start the operation again, or he will be unaware of the malfunction, but the combustible mixture will still be escaping.
The shutter may remain open without the user's knowledge or under his control. In this case the flame burner will remain in operation, without thereby igniting the catalytic burner, the entire combustible mixture being burnt in the region of the flame burner. In this case, on the one hand, the catalyst may be permanently damaged because of the overheating to which it is subjected and, on the other hand, the flames escaping through the orifices in the junction tube can heat in a damaging manner the components on which work is being carried out, for example an electronic circuit.